Resistance element for rheostats



June 20, 1939. A. w. PLENSLER v RESISTANCE ELEMENT FOR RHEOSTATS FiledMay 13, 1935 P06715710, I a (AM.

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Patented June 20, 1939 PATENT OFFICE RESISTANCE ELEMENT FOR RHEOSTATSAlexander W. Plensler, Chicago, Ill., assignor to 'BesistellteCorporation, Chicago, 111., a corporation of Illinois Application May13, 1935, Serial No. 21,080

4 Claims.

7 This invention relates generally to rheostats or the'like and moreparticularly to such devices embodying improved resistance elementsproduced by a novel, simple and economical process.

, An'object of the invention is to provide a resistance element ofimproved form and novel construction which is especially suited forrheostats and like devices for use in radio apparatus and which permitsthe rheostats to be constructed in exceedingly compact form and'yetprovide smooth, uniform and accurate resistance control.

Another object resides in the provision of an improved resistanceelement by a novel process, which element is relatively inexpensive toproduce, is durable, wear resisting, unaflected by temperature orhumidity variations, and which retains its original resistancecharacteristics indefinitely.

Another object is to provide a novel process of producing resistanceelements whereby substantial uniformity of resistance characteristicswithin narrow tolerance limits may be obtained under quantity productionmethods.

Anotherv object is to provide a novel process of producing resistanceelements wherein the element is built up of relatively thin sheets ofre- 'sistance material coated-with a thin film of varnish, for example,'an uncured phenol condensation product which, when cured, binds thesheets together to form a homogeneous, compact mass having uniformresistance characteristics throughout.

Another object is to provide a novel process of producing resistanceelements embodyinga compact, homogeneous mass of resistance materialwherein a relatively large body or block of material having the desiredresistance characteristics is built up in laminated form. and the Iindividual elements are formed fromthis block 40 by simplemachiningoperations.

A further object is .to provide an improved rheostat having a minimumnumber of simple and durable parts, which is exceedingly compact inconstruction, and which is well adapted for very accurate and uniformcontrol of electrical circuits.

' Other objects and advantages of the invention will be apparent fromthe following description taken in connection with the accompanyingdrawing in-which the various elements entering into a preferredembodiment of the invention are shown in enlarged form. y

In the drawing: I

Figure 1 is a vertical, longitudinal, sectional view through a rheostatembodying the features sistance material built up in accordance with theof the steps in the manufacture of the resistance element shown in Figs.1 and 2.

Fig. 4 is a perspective view of a strip of the I material entering intothe construction of the block shown in Fig. 3 from which the resistanceelements are formed.

While the invention is susceptible of various modifications andalternative constructions, I have shown in the drawing and will hereindescribe in detail a preferred embodiment, but it is to be understoodthat I do notthereby intend to limit the invention to the specific formdisclosed, but intend to cover all modifications and alternativeconstructions falling within the spirit and scope of the invention asexpressed in the appended claims.

To illustrate the invention, I have shown in Fig. 1 of the drawing arheostat particularly adapted for use in remote control units such asare commonly employed with automobile radio sets wherein compactness ofconstruction is very desirable and smooth and accurate resistancecontrol essential. As shown in the drawing, the rheostat maybe enclosedwithin and forms an integral part of a metal casing 5 of the controlunit, thus requiring a minimum of space and materially simplifyingconstruction.

The rheostat comprises generally a resistance element 6 and means forconnecting varying portions of theelement in an electrical circuit. In

its preferred form as shown in Figs. 1 and 2, the.

posed transversely of the axis of the element. A

central aperture 1 adapted to receive a contact 8 in sliding engagementwith the edges of the laminae provides a connection to one side of thecircuit (conveniently the grounded side). A connection to the other sideof the circuit is provided by a spring contact 9 adapted to be held insurface engagement with the base of the element 6;

To support the various elements of the rheostat, the casing 5 is formedwith a. suitable cavity or bore having an intermediate enlargement whichconstitutes a receptacle for the resistance unit 6. The enlargement ispreferably of slightly greater diameter than the element to providespace for a sleeve 9 of any suitable insulating material for preventingelectrical contact between the sides of the element and bore. The innerend of the element 6 is preferably insulated from the casing 5 by meansof an insulating element III which may conveniently be formed integrallywith the element. The resistance element is thus enclosed within thecasing 5 but is insulated therefrom and the outer'end is disposed forconvenient engagement by the spring contact 9.

of the cup ll.

To facilitate assembly of the rheostat and toprovide for readilyconnecting the same in circuit, the spring contact 9 may conveniently beincorporated in a removable plug assembly. As

shown herein, the contact spring 9 is of generally U-shaped form and iscarried in a shallow cup ll of suitable insulating material. One leg ofthe spring 9 is bent upwardly so as to project above the rim of the cupI l and the other leg rests upon the bottom of the cup and is solderedor otherwise suitably secured to the end of a conductor l2 projectingthrough the bottom of the cup. To hold the spring firmly in place, awasher l3 engaging the outer surface of the cup and soldered 5 to theconductor l2 may be provided.

To hold the cup H and spring contact 9 in position in the rheostat, theplug assembly includes a sleeve member l4 threading into the bore in thecasing 5 and bearing against the bottom Conductor l2 which leads to theradio set or circuit to be controlled extends rearwardly through thesleeve I4. To protect the conductor from such contact with the sleeve aswould tend to wear off and weaken the insu- 2 lation, a bushing I5 isprovided at the outer end of the sleeve. The bushing I5 may be formed ofany suitable material such as soft rubber which further prevents theconductor from being bent sharply at the point of leaving the sleeve and30 thus adds to the life of the equipment. I

Sliding contact by which the resistance of th element 6 is variablyincluded in the electrical circuit is,provided by the contact member 8.As shown herein, the contact member 8 is in the 5 form of a split metalpin formed integrally with or suitably secured to a screw l6 and adaptedto project into the aperture in the element 6. The armsof the pin flareoutwardly so as to yieldably engage with the cylindrical surface of theaper- 0, ture, the resiliency of the metal holding them in firmelectrical engagement therewith. The ends of the legs may be rounded tominimize wear on the element.

The screw l6 which carries the contact mem- 5.1381 8 is adapted tothread into the bore in the 7 rear of the element increases the gapbetween the member and the contact spring 9, thus providing a longerconductive path through the element with a resultant increase inresistance while a reverse movement reduces the circuit resist- Q0 ance.The threads of the screw [6 may, of course,

be formed with any desired lead so as to provide the requisitesensitivity of adjustment.

The screw I 6 provides electrical connection with the casing 5 and thuswith the other side 55. of the circuit. Ordinarily, the casing will bemounted upon a metal part of the car which, in the usual practice, isgrounded to one side of the battery. This connection is illustrateddiagrammatically by'a conductor -|8 shown as at- 70 tached to the casing5. It will be appreciated that this conductor may be connected at anypoint on the frame with which the casing is in contact.

The compact construction, uniform and accurate resistance control aswell as the sensitivity to and reliability of the rheostat is madepossible not only by the simple construction above described, but by theimproved resistance element 6 which is a product of the novel process.of manufacture contemplated by the invention. It will be apparent thatthe element 6 may be made in any desired shape but that the cylindricalform is to be preferred as it lends itself most readily to efiicientmanufacture both with respect to the element itself and to theassociated parts which,

. with the element, constitute the completed de-- ments. In the case ofresistance elements formed of a mass of finely divided or comminutedmaterial, it is exceedingly difficult to meet these requirements withany degree of accuracy. Thus, there is not only the problem of providinga resistance element which will meet particular circuit requirements butalso of producing the elements with a reasonable degree of uniformity.In quantity production, particularly, uniformity within quite narrowtolerance limits is essential but difficult to obtain.

By the present process it is a relatively simple I matter to produceresistance elements having any desired resistance characteristics.Moreover, it is possible to produce resistance elements in largequantities which elements are of substantially uniform character,variations being held within materially closer limits than hasheretofore been practical. Briefly, the present process contemplates theproduction of resistance elements in laminated form, the resistancematerial being first incorporated in relatively thin sheets which sheetsare then united by means of a suitable binder to form a homogeneousblock of resistance material having the desired resistancecharacteristics. The individual elements are then produced from thisblock by simple machining operations.

Referring more particularly to the process of forming the block ofresistance material from which the elements are produced, the laminae orsheets from which the block is formed preferably comprise sheets ofabsorbent material carrying in intimate association finely dividedparticles of resistance. material. The sheets may be formed by immersinga sheet of suitable absorbent material in a bath containing finelydivided particles of a selected resistance material under suitablycontrolled temperature and other conditions.

Many types of absorbent material may be used, a good grade of absorbentpaper being particularly suitable. For the sake of uniformity, the papershould be of even texture throughout so that the finished product willbe substantially .free of variations. Likewise, any suitable resistancematerialmaybe .used, a preferred material being graphite. This material,in finely divided form, is

' held in suspension preferably as a colloid, in a suitable fluid suchas water; the concentration depending upon the amount of material whichthe sheet is to take up. Upon the sheet being immersed in the bath, thesuspended material is uniformly distributed through itsfibers and isheld therein when the sheet is removed. Thus, a sheet may be producedwith any desired resistance characteristics and may readily beduplicated within close limits.

After treatment as above described, the sheet is carefully dried andthen coated with a thin varnish, preferably an uncured phenolcondensation product. The varnish is applied in suillcient amount tobind a graphite coated sheet to another sheet without, however,insuating them from one another. A partially coate sheet is shown inFig. 4 by way of illustration. In that figure, the reference characteris designates a coated portion of the sheet having the finely dividedparticles of resistance material uniformly distributed throughout whilethe thin coat of varnish thereon is indicated at 20.

The next step in the process is to build up the block of resistancematerial from the coated sheets above described. Accordingly, a numberof sheets carrying the proper amount of resistance material to give thedesired results are placed one on top of another to form a stack of therequired dimensions. To produce resistance elements having uniformresistance characteristics. throughout, sheets carrying substantiallyidentical quantities of resistance material will be employed. To produceelements with a taper", sheets having varying quantities of theresistance material will be used and these will be arranged in theproper sequence necessary to give the desired resistance characteristicsto a completed unit.

Upon the top of the stack of coated sheets a final untreated sheet 2! offiber may be placed which is coated with the varnish but which carriesno resistance material. This sreet is employed where it is desirable toinsulate one end of the resistance elements from their supportingstructure as in the instance hereinbefore described. The entire stack ofsheets is then cured under heat and pressure; the varnish binding thesheets together into a substantially solid mass or block 22 throughwhich the finely divided particles of resistance material are uniformlyand conductively dispersed.

After curing, the block 22 may be formed into resistance elements'in anyconvenient manner. As herein shown, apertures 1 are first formed in theblock 22 as by drilling, the apertures being spaced to providesufficient material from which to form the elements ,6. The block isthen cut into substantially square sections 23, each section vconventional manner.

having one of the apertures 1 included therein. The square sections 23may, of course, be used in this form, if desired, or they may be turnedto the cylindrical form shown in Fig. 2. In producing the cylindricalelements it has been found convenient to mount the square sections uponan arbor held in a lathe and to turn them down in a The elements arethus produced with an accurately centered aperture 1 so that the laminaecomprise substantially annular sheets of resistance material. Theelements may readily be formed to exact dimensions which, with thecarefully controlled process of forming the material, makes it possibleto obtain an unusual degree of uniformity in elements produced underquantity production methods.

In view of the foregoing, it will be apparent that and uniformresistance control.

the resistance element 0 formed in the above manner presents a smooth,hard, wear resisting surface for engagement by the sliding contact Thusthe element is durable and its resistance characteristics remainunchanged in use. Moreover, these characteristics are unaffected bytemperature or humidity variations. The element does not produceelectrical noise in use and is therefore especially suitable for radiouse. Its homogeneous character permits very accurate Further, rheostatsemploying the improved elements may be constructed in extremely compactform without sacrificing accuracy or other desirable characteristics.

It will also be apparent that I have devised a novel process ofproducing resistance elements by which process the elements can bereadily made to any desired standard. Further, the resistance elementsproduced by this process are of substantially uniform character whichgreatly facilitates their manufacture in commercial quantities.

I claim as my invention:

1. An elongated resistance element having a longitudinally extendingcontact surface, said element comprising a plurality of sheets ofabsorbent material carrying uniformly dispersed finely dividedresistance material, said sheets being disposed transversely with theedges thereof providing said contact surface and bonded together toproduce a substantially solid mass in which the sheet edges form saidcontact surface.

2. An elongated resistance element having a longitudinally extendingcontact surface, said element comprising a plurality of sheets ofabsorbent material carrying uniformly dispersed finely dividedresistance material, said sheets being stacked in face-to-face relationand disposed so that the edges of the sheets present a contact surface,a sheet of insulating material disposed at one end of the stack ofabsorbent sheets to form an insulating terminal for one end of theelement, and means bonding said absorbent sheets and said insulatingsheet together to form a substantially solid mass.

3. A resistance element comprising a plurality of sheets of absorbentmaterial impregnated with finely divided particles of resistancematerial, said sheets being substantially annular in form and disposedin surface-to-surface contact to define an elongated tubular elementwith the edges of the sheets defining a contact surface extendinglongitudinally of the element, and

means permanently binding said sheets together edge portions of thesheets providing for current conductivity to the sheets segments of theelement.

ALEXANDER W. Pm

